USING HVSR TO REMAP OHIO’S BEDROCK TOPOGRAPHY AND DRIFT THICKNESS

Over two-thirds of Ohio’s surface was once covered by glaciers, and the elevation of the buried bedrock surface (bedrock topography, BT) and the thickness of unconsolidated materials (drift thickness, DT) in these areas are fundamental geologic datasets that address subsurface modelling, resource management, and construction concerns. Previously produced BT and DT models compiled by the Ohio Geological Survey (OGS) in the early 2000s are often inadequate for many detailed applications, as they were created with occasionally sparse control points of variable quality. The Horizontal-to-Vertical Spectral Ratio (HVSR) technique has become an important tool to refine mapping in Ohio by facilitating relatively rapid and efficient unconsolidated material thickness estimates for an area. These methods and more accurate data have led to the development of a long-term mapping program dedicated to updating the BT and DT datasets of Ohio. During the past several years, the HVSR technique supported the near-surface mapping of over 20 published and unpublished OGS products, primarily 7.5-minute quadrangles and county-scale maps. The HVSR-driven approach to remapping BT and DT has led to the collection, processing, and interpretation of about 3,500 HVSR data points between the 2017–2021 field collection seasons.

While an improvement over historical methods in BT and DT mapping in Ohio, the use of HSVR data includes uncertainty due to the nature of calibration wells, geologic settings, and geophysical data processing. Preliminary statistical analyses support a theoretical, calculated average uncertainty of about 21% for HVSR depth estimates within a 7.5-minute quadrangle-based study area in Willshire, OH-IN. However, the updated DT/BT model created with HVSR data compared to one recently drilled, deep (335ft) rotosonic core to bedrock within the same study area suggested that the practical percent error may be as low as 2%. This result suggests that the predictive power of an HVSR-derived depth estimate is largely dictated by how appropriate a developed calibration equation fits the geologic setting, even if the power curve inherently contains notable uncertainty. However, continued ground truthing and cautious data analysis for more study areas in Ohio is needed to better understand the accuracy of the HVSR method.